Vector modulator

ABSTRACT

An analogue phase shifter with vector modulator is envisioned. The analogue phase shifter has an input stage that provides the generation of a pseudo-basis of four vectors. The analogue phase shifter has a multiphase filter. The analogue phase shifter has an output stage that controls the amplitde of the basis vectors, and acts to recombine them. The input stase is made with a phase opposition signal generator and a multiphase filter. The input stase is made of four variable attenuators. The attenuators are for separate control of the amplitude of each of the basis vectors. The output stage also has a common output providing the summation of four quadrature channels. In one case, the output stage includes two differential amplifiers. In another case the quadrature channels are connected to the common output through buffer amplifier stages, each followed by a summation capacitor.

This application is a national phase of PCT/FR99/01660 which was filedon Jul. 8, 1999, and was not published in English.

DESCRIPTION

1. Technical Field

The present invention concerns a vector modulator.

2. State of Prior Art

Electrical signal phase shifting may be digital, with phase jumps, oranalogue, with continuous phase variation.

Digital phase shifters use PIN-diodes or field effect transistors, usedas electronic switches, allowing switching between transmission lines ofdifferent lengths (phase shifting by propagation/transmission delay) orelse between sections of filtering structures of the low-pass/high-passtype, for example.

The use of varactor diodes or field effect transistors as continuouslyvariable impedances allows analogue phase shifters to be made, providingwave phase shifting in transmission, as in vector modulators, or else inreflection, as in diode phase shifters.

In a diode phase shifter, as shown in FIG. 1, the phase and quadratureoutputs of the hybrid coupler 10 are charged by variable reactiveelements. The variation in reactive impedance relative to thecharacteristic impedance (generally 50Ω) entails a variation in thephase of the output signal S by means of a complex reflectioncoefficient {tilde over (Γ)}.

The following signals are obtained: $\begin{matrix}{{Input}\text{:}} & {\sin \quad \left( {\omega \quad t} \right)} \\{{Output}\quad O\text{:}} & {\overset{\sim}{\Gamma}\quad \sin \quad \left( {\omega \quad t} \right)} \\{{Output} - {90{^\circ}\text{:}}} & {{- \overset{\sim}{\Gamma}}\quad \cos \quad \left( {\omega \quad t} \right)} \\{{Output}\quad S\text{:}} & {\left. {{{- \overset{\sim}{\Gamma}}\quad \cos \quad \left( {\omega \quad t} \right)} = {{{\overset{\sim}{\Gamma}}\quad \sin \quad \left( {\omega \quad t} \right)} + \phi}} \right),}\end{matrix}$${{with}\quad \tan \quad \left( \frac{\phi}{2} \right)} = \frac{1 - \Gamma}{1 + \Gamma}$

Diode phase shifters essentially use voltage controlled varactors forcontinuous phase shifting, or switch operated PIN-diodes for digitalphase shifting.

In principle, hybrid ring or square couplers or transformers may equallywell be used. The band width of the phase shifters thus constituted islimited by the use of a coupler: these devices are typically narrowband, or operate at best on a two-octave band: ω→4ω.

Vector modulators use the principle of addition of variable amplitudeorthogonal vectors (I/Q).

In a vector modulator phase shifter, phase shifting between the inputsignal E and the output signal S is the result of recombining (11) twoseparately attenuated quadrature components. An example of a 0-90° phaseshifter with vector modulator is given in FIG. 2, with I and Q controls.

The record of the signals taken at the different points of the phaseshifter is as follows: $\begin{matrix}{{Input}\quad E\text{:}} & {\sin \quad \left( {\omega \quad t} \right)} \\{{Output}\quad {O{^\circ}}\text{:}} & {\sin \quad {\left( {\omega \quad t} \right)/\sqrt{2}}} \\{{Output}\quad - {90{^\circ}\text{:}}} & {{\sin \quad {\left( {{\omega \quad t} - {\pi/2}} \right)/\sqrt{2}}} = {{- \cos}\quad {\left( {\omega \quad t} \right)/\sqrt{2}}}} \\{{Point}\quad A\text{:}} & {a\quad \sin \quad {\left( {\omega \quad t} \right)/\sqrt{2}}} \\{{Point}\quad B\text{:}} & {{- b}\quad \cos \quad {\left( {\omega \quad t} \right)/\sqrt{2}}} \\{{Output}\quad S\text{:}} & {{\left\lbrack {{a\quad \sin \quad \left( {\omega \quad t} \right)} + {b\quad \cos \quad \left( {\omega \quad t} \right)}} \right\rbrack/\sqrt{2}} = {\cos \quad \left( {{\omega \quad t} + \phi} \right)}}\end{matrix}$

if a=cos φ and b=sin φ

These arrangements providing phase shifting over one quadrant, a 0-360°phase shifter is obtained by combining 90° hybrid couplers 10 and 180°hybrid couplers 12, as shown in FIGS. 3A, 3B or 3C, or else by cascadingfour 0-90° cells. The circuit bearing the reference 13 is a ¼ phasesplitter and the circuits bearing the reference 14 are 4:1 phasecombiners.

These different phase shifters are all based on the generation of phasequadrature signals, by using and combining 90° hybrid couplers. Onprinciple, the 90° hybrid couplers have a band width extending at mostto about two octaves. It is therefore the same for the phase shiftersthus constituted.

The article bearing the reference [1] at the end of the descriptiondescribes structures of this type and particularly that of a vectormodulator.

The object of the invention is to overcome the drawbacks of the priorart arrangements by proposing a new type of vector modulator.

DISCLOSURE OF THE INVENTION

The present invention concerns a vector modulator, characterised in thatit includes:

an input stage providing the generation of a pseudo-basis of fourvectors (±I, ±Q), and comprising a multiphase filter;

an output stage making it possible to control the amplitude of the basisvectors, and to recombine them.

The input stage comprises successively:

a phase opposition signal generator;

a multiphase filter or network.

The output stage comprises:

four variable attenuators making it possible to achieve separate controlof the amplitude of each of the basis vectors;

a common output providing the summation of the four quadrature channels.

To advantage the output stage includes two differential amplifiersisolating the variable attenuators from the multiphase filter/network.The quadrature channels are connected to the common output throughbuffer amplifier stages each followed by a summation capacitor.

The potential applications for this type of vector modulators arevaried:

formation of beams (antenna arrays, antenna and synthetic aperture radaretc.);

high speed communications, frequency and angular multiplexing (satellitecommunications, digital television, WDM switchboard (see documentsbearing the references [2] and [3] at the end of the description) etc.);

instrumentation (correlator, phase discriminator, vector analyser, etc.)

Such a modulator may be used, in fact, for the transmission of singleside-band (SSB), or even suppressed carrier single side-band signals.Such transmission modes are appropriate for satellite communications,digital television, or telephony. The principle consists in transposingthe frequency of the signal wanted to encode a carrier, so as to meetelectromagnetic spectrum congestion or transmission quality criteria. Itmay also be used in antenna arrays or synthetic aperture radar: beamscanning by phase scanning.

In the case of digital television, there are two successive encodings:encoding the video image, then carrier modulation to 12 Ghz etc. Byextrapolating such an embodiment, an analogous embodiment in optics maythus be conceived: hence the WDM modulation.

However, applications, functions or systems exploiting phase quadraturesignals in a very wide frequency band (correlator, network analyser,phase discriminator, etc.) are particularly attractive.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 shows a prior art 0-90° phase shifter with varactors;

FIG. 2 shows a prior art 0-90° phase shifter with vector modulator;

FIGS. 3A, 3B and 3C show prior art 0-360° phase shifters by vectormodulator;

FIG. 4 shows the vector modulators according to the invention;

FIGS. 5A and 5B show respectively a multiphase network structure with nsections, and the quadrature deviation between the I/Q vectors as afunction of the reduced frequency f×2πRC for n variant of 1 to 5sections;

FIGS. 6A and 6B show respectively the transfer function S21 of fourbasis vectors +I, −I, +Q, −Q, and the phase error relative to a perfectquadrature between the vectors I and ±Q, and a perfect phase oppositionbetween the vectors +I and −I, obtained on a vector modulator comprisinga four section multiphase filter.

DETAILED DISCLOSURE OF THE EMBODIMENTS

The present invention is based on the principle of a multiphase filtervector modulator, as shown in FIG. 4.

This modulator comprises:

an input stage 18 providing the generation of a pseudo-basis of fourvectors (±I, ±Q);

and an output stage 19 making it possible to control the amplitude ofthe basis vectors, and to combine them.

The input stage 18 comprises successively:

a generator 20 of phase opposition signals +v(t) and −v(t), which may beany device supplying two signals out-of-phase by 180°: a transformer, a180° hybrid coupler. An advantageous solution uses a one or severalstage differential amplifier, for its large band width and for accuracyin obtaining signals out-of-phase by 180°;

a multiphase filter or network 21.

The multiphase filter/network structure, as shown in FIG. 5A, is used inthe field of audio-frequencies (−300-3000 Hz), to make single side-bandmodulators. It consists preferentially of a cyclical and repetitivestructure based on equal resistors (R) and capacitors (C, C/2toC/2^(i−1), to C/2^(n−1)) decreasing geometrically, as shown in FIG. 5A.

The multiphase filter/network 21 output delivers a pseudo-basis of fourorthogonal vectors in twos (±I, ±Q).

The multiphase filter/network 21 frequency response may be described bya characteristic frequency which shows the low frequency of use:f₀=½πRC.

The band width for a given quadrature error 80 increases with the numberof sections.

FIG. 5B shows the quadrature deviation between the I and Q vectors as afunction of the reduced frequency f×2πRC for n variant of 1 to 5sections.

The output stage 19 comprises:

four variable attenuators 22, 23, 24 and 25 making it possible to obtainseparate control of the amplitude of each of the basis vectors; toadvantage these attenuators 22, 23, 24 and 25 may be isolated from themultiphase filter/network 21 by two differential amplifiers 26 and 27,so as to minimise the influence of the variations in charge on theoverall behaviour of the modulator;

a common output S providing the summation of the four quadraturechannels; these four quadrature channels may thus be connected to thecommon output port S through buffer amplifier stages 30, 31, 32 and 33followed by summation capacitors 34, 35, 36 and 37.

The output stage 19 may comprise equally well any structure making itpossible to modify separately the amplitude of the basis vectors(mixers, variable gain amplifiers for example).

FIG. 6A shows the transfer function S21 when each of the four basisvectors +I, −I, +Q, and −Q is successively selected. Measurement iscarried out between 10 MHz and 3000 MHz. The band width at −3 dB of eachof these vectors is of the order of 30 MHz-1500 MHz (that is a ratio of50 to 1), which is also the band width of the vector modulator. Itsoperating band for a given phase error (for example ±5°) is slightlylower (˜80 MHz-1300 MHz at ±5°).

FIG. 6B shows the phase error relative to a perfect quadrature between Iand ±Q, and to a perfect phase opposition between I and −I.

REFERENCES

[1] <<Microwave Components and Subsystems>> (Anaren, Proven Performancein Signal Processing, pp.108-115 and 125-129)

[2] <<Advanced Technologies Pave The Way For Photonic Switches>> by RodC. Alferness (Laser Focus World, February 1995, pp.109 to 113)

[3] <<Wavelength-Division Multiplexing Technology in PhotonicSwitching>> by Masahiko Fujiwara and Shuji Suzuki (<<Photonic Switchingand Interconnects>> by Abdellatif Marrakchi, Marcel Dekker, Inc., pp.77-113)

[4] <<The Art of Electronics>> by Paul Horowitz and Winfield Hill(Cambridge University Press, Second Edition, 1989, chapter 5: <<ActiveFilters and Oscillators>>, FIG. 5.41)

[5] <<Radio Amateur's Handbook>> by Frederick Collins and RobertHertzeberg (15^(th) ed. Rev., 1983, pp.12-8 and 12-9)

What is claimed is:
 1. An analogue phase shifter with vector modulatorcomprising: an input stage, providing the generation of a pseudo-basisof four vectors and comprising a multiphase filter; an output stage, forcontrolling the amplitude of the basis vectors, and to recombine them;the input stage comprising: a phase opposition signal generator; and amultiphase filter; the input stage comprising: four variable attenuatorsfor separate control of the amplitude of each of the basis vectors; acommon output providing the summation of four quadrature channels; andwherein the output stage includes two differential amplifiers isolatingthe variable attenuators from the malliphase filter.
 2. An analoguephase shifter with vector modulator comprising: an input stage,providing the generation of a pseudo-basis of four vectors andcomprising a multiphase filter; an output stage, for controlling theamplitude of the basis vectors, and to recombine them; the input stagecomprising: a phase opposition signal generator; and a multiphasefilter; the input stage comprising: four variable attenuators forseparate control of the amplitude of each of the basis vectors; a commonoutput providing the summation of four quadrature channels; and whereinthe quadrature channels are connected to the common output trough bufferamplifier stages, each followed by a summation capacitor.